The mTOR pathway is a signaling system that regulates growth and metabolism in response to the nutritional state of the organism. Increasing evidence indicates that the pathway is commonly deregulated in cancer, neurodegeneration, and diabetes, and also plays a major role in the aging process. The large mTOR protein kinase is the target of the drug rapamycin and the catalytic subunit of two multi-protein complexes, mTOR Complex 1 (mTORC1) and 2 (mTORC2) that nucleate distinct branches of the mTOR pathway and respond to different upstream signals. mTORC1 responds to a diverse set of stimuli, such as growth factors, nutrients, and stresses, and regulates many anabolic and catabolic processes, including protein and lipid synthesis and autophagy. Recently, we discovered that mTORC1 regulates, in a non-cell autonomous fashion, the self-renewal of intestinal stem cells (ISCs) in response to caloric restriction (CR). mTORC1 acts in Paneth cells, which constitute the niche for ISCs and are located at the base of intestinal crypts. CR is a reduction in caloric restriction in the absence of malnutrition and has very interesting effects in mice, such as decreasing tumor growth and increasing lifespan. The mechanisms through which CR functions are not well understood in mammals. The broad goals of our work are to arrive at a mechanistic understanding of how mTORC1 senses the CR state in Paneth cells, how its activity modulates Paneth cell function to regulate ISCs, and to determine the implications of our work for understanding the effects of CR on tumorigenesis. The specific aims of our proposed work are to: identify the mTORC1-dependent effectors through which CR acts in Paneth cells to promote ISC self renewal (Aim 1); determine the factors Paneth cells use to modulate intestinal ISC renewal in response to CR (Aim 2); and determine how CR and mTORC1 activity in Paneth cells regulate intestinal tumorigenesis (Aim 3). We will accomplish our goals with a multi- disciplinary approach that uses the tools of biochemistry, molecular biology, and mouse engineering. Our results are likely to have important consequences for our understanding of the clinically important mTOR pathway. Moreover, the signaling mechanisms we uncover may serve in the future as targets for the development of therapies that mimic some of the beneficial effects of CR.